The harvest is done in two stages:
And I looked, and behold a white cloud, and upon the cloud one sat like unto the Son of man, having on his head a golden crown, and in his hand a sharp sickle. And another angel came out of the temple, crying with a loud voice to him that sat on the cloud, Thrust in thy sickle, and reap: for the time is come for thee to reap; for the harvest of the earth is ripe. And he that sat on the cloud thrust in his sickle on the earth; and the earth was reaped.
And another angel came out of the temple which is in heaven, he also having a sharp sickle. And another angel came out from the altar, which had power over fire; and cried with a loud cry to him that had the sharp sickle, saying, Thrust in thy sharp sickle, and gather the clusters of the vine of the earth; for her grapes are fully ripe. And the angel thrust in his sickle into the earth, and gathered the vine of the earth, and cast it into the great winepress of the wrath of God. And the winepress was trodden without the city, and blood came out of the winepress, even unto the horse bridles, by the space of a thousand and six hundred furlongs.
- Revelation 14:14-20 (KJV)
The order of the harvest is as follows:
1. The believers
2. The unbelievers
The harvesting is done by the angels (reapers), and the unbelievers will be destroyed by fire.
The harvest from the book of Revelation, is similar to the harvest from Matthew 13.
Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
- Matthew 13:30 (KJV)
Although it seems that the unbelievers are harvested first, they are actually bundled to be burned later, so the believers are still harvested first.